Occam's razor (sometimes spelled Ockham's razor) is a principle attributed to the 14th-century English logician and Franciscan friar, William of Ockham.

The principle states that the explanation of any phenomenon should make as few assumptions as possible, eliminating those that make no difference in the observable predictions of the explanatory hypothesis or theory.

The principle is often expressed in Latin as the lex parsimoniae ("law of parsimony" or "law of succinctness"): "entia non sunt multiplicanda praeter necessitatem", roughly translated as "entities must not be multiplied beyond necessity". An alternative version "Pluralitas non est ponenda sine necessitate" translates "plurality should not be posited without necessity".

This is often paraphrased as "All other things being equal, the simplest solution is the best." In other words, when multiple competing theories are equal in other respects, the principle recommends selecting the theory that introduces the fewest assumptions and postulates the fewest entities. It is in this sense that Occam's razor is usually understood. This is, however, incorrect. Occam's razor is not concerned with the simplicity or complexity of a good explanation as such, it only demands that the explanation be free of elements that have nothing to do with the phenomenon (and the explanation).